Well they both have advantages but in my opinion Devolution. For one thing it doesn't install to the system NAND so it's very simple to setup and delete if you decide you don't want it anymore. It also supports disc audio streaming which Dios Mios doesn't and runs the games in Wii Mode so you don't have to turn off your system to exit the loader.

Well if it says that it supports dual disc then it should, I'm right? I'm especially interested in Symphonia as well.

It definitely does support dual disc, yes. But all I know about how to do it is what was stated by Devo's author "place disc2.iso in the same folder as disc one and press eject when prompted to load the 2nd disc."